Conventionally, a technique for providing mobility of the entire network composed of a movable terminal group and a router has been examined in IETF NEMO WG.
NEMO (Network Mobility) described in the document Network Mobility (NEMO) Basic Support Protocol (written by Vijay Devarapalli et al., draft-ietf-nemo-basic-support-02.txt) is a technique that implements data communication between a node in a “mobile network” and a node in an external network, by means of the mobile network and a home agent that manages the position of the mobile network, where “mobile network” refers to a network composed of the movable terminal group, and a mobile router connecting to an external network such as the Internet.
FIG. 24 illustrates a conventional mobile network system disclosed in the document Network Mobility (NEMO) Basic Support Protocol. 
In FIG. 24, terminal 8 connected to home network 2 connects to global network 1 such as the Internet through home gateway (HGW) 12 and can communicate with plural terminals 9 connected to global network 1.
Mobile routers (MR) 5, 6, 7 connected to mobile networks 3, 4 connect to global network 1 through access router (AR) 11.
In this case, the mobile networks need to be composed as respectively independent networks, and thus the network prefix of home network 2 (Pref_Home), that of mobile network 3 (Pref_NEMO3), and that of mobile network 4 (Pref_NEMO4) need to be set differently from one another. Here, these prefixes are assumed to be Pref_Home (1:1:1:/48), Pref_NEMO3 (1:1:1:3/64), and Pref_NEMO4 (1:1:1:4/64), respectively.
When connecting to access router (AR) 11, mobile routers 5, 6, 7 acquire network prefix information advertised by access router 11, or issue a request to the DHCP server (not illustrated), to acquire a care-of address (CoA), and then register the address in home agent (HA) 10.
Here, when terminal 8 disconnects with home network 2 and moves to mobile network 3, terminal 8 combines Pref_NEMO3 included in router information advertised from mobile router 5 or 6, and terminal address (assumed to be 0:0:0:8) to create address 1:1:1:3::8.
After then, when a packet addressed to terminal 8 positioned at address 1:1:1:3::8 arrives at home network 2, home agent 10 acquires the packet. Then, home agent 10 transfers the packet acquired to mobile router 5 or 6. Home agent 10 encapsulates the data addressed to terminal 8 to perform tunneling transfer between mobile router 5 or 6 and home agent 10.
In this way, while connecting to home network 2, mobile routers 5, 6, 7 relay a communication packet addressed to the terminal by use of Home address (HoA) within mobile networks. While connecting to access router 11, home agent 10 relays a communication packet addressed to terminal 8 within the mobile networks 3, 4.
This process causes a communication packet addressed to terminal 8 to be transferred from home agent 10 to mobile routers 5, 6, 7 by tunneling, even if a mobile network that terminal 8 connects to moves during communication between terminal 8 and terminal 9, and connects to access router 11 different from a HGW. Consequently, communication between terminal 8 and terminal 9 is maintained without interruption.
With the above-described conventional configuration, however, the address of terminal 8 remains to be 1:1:1:3::8 even if terminal 8 that has been connected to mobile network 3 moves alone to connect to mobile network 4, and thus a packet addressed to terminal 8 results in being sent to mobile network 3.
In order to prevent this problem, terminal 8 needs to reacquire an address (e.g. 1:1:1::8 for home network 2) when moving to another network. Then, terminal 8 needs to perform a process such as changing DNS settings by means of Dynamic DNS or the like when reacquiring an address. If terminal 8 fails to do it, home agent 10 becomes unable to transmit a packet to terminal 8. However, even if using Dynamic DNS, the address is changed when terminal 8 moves between networks, disconnecting a session between terminal 8 and terminal 9. Further, it is not practical that all the terminals implement Dynamic DNS because of problems such as cost.